The extraction of metals from aqueous solutions, particularly those aqueous solutions containing, in addition thereto, other metals which are of less value, is well known. In fact, the recovery of zinc, cadmium, nickel, copper, iron, cobalt, etc. utilizing phosphorus containing extractants has been known for many years, see U.S. Pat. No. 3,966,569.
More recently, in U.S. Pat. Nos. 4,348,367 and 4,619,816 cobalt has been recovered from aqueous solutions, free of or containing nickel, with various phosphinic acids such as bis(2,4,4-trimethylpentyl) phosphinic acid. U.S. Pat. No. 4,382,016 discloses a composition for use in the extraction comprising an inert material and an organophosphinic acid while U.S. Pat. No. 4,374,780 is also pertinent in said separations. U.S. Pat. No. 4,721,605 discloses the extraction of zinc, silver, cadmium, mercury, nickel, cobalt and copper from aqueous solution utilizing dithiophosphinic acids or their salts.
Thus, it can be readily appreciated that many phosphinic acid derivatives have been found to be excellent extractants for the recovery of variety of metals from aqueous solutions containing said metals alone or in combination with other, less desirable metals.
It has been recognized that the phosphinic acids of U.S. Pat. No. 4,382,016 are less acidic than those dithiophosphinic acids of U.S. Pat. No. 4,721,605, i.e., the acidity of the acids increases with increasing sulfur content of the molecule. Because the strongest acids, i.e., the dithiophosphinic derivatives function at lower pHs they provide superior separations from various low pH effluent streams, however, their strength also is a detriment because it renders the extracted metal more difficult to strip therefrom. Thus, while excellent extraction of zinc, for example, is achieved using the dithiophosphinic acids, the recovery of the zinc from the extractant phase is very difficult thereby rendering the overall process of using the dithiophosphinic acids more costly. While many industries can tolerate the higher cost of using the dithiophosphinic acids, certain industries where the savings resulting from the metal recovery is small, would prefer to use extractants from which the extracted metal is more easily recovered.
Therefore, if an extraction process would be devised which would enable complete extraction of metals from aqueous solution and a less expensive metal recovery from the extractant phase, a long felt need would be satisfied.